These pics might help. The carb has an Idle jet that is often missed. Go to this link at look at Vespa Carb Cleaning. You will see the idle jet in Pic # 10. This may need to be cleaned. DON"T drill it out, use a very small piece of wire (strand from control cable) and c\blow it out with compressed air.
Here is the link; http://briefcase.yahoo.com/bwebb08
go to Vespa Carb Cleaning folder